Hydraulic machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy equipment, use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to hydraulic pumps on the machine that provide pressurized fluid to chambers within the actuators. The hydraulic pumps force pressurized fluid to move into or through the chambers such that the pressure of the fluid acts on hydraulic surfaces of the chambers to move the actuator and/or a connected work tool. When the pressurized fluid is drained from the chambers it is returned to a fluid storage tank on the machine for reuse.
One problem associated with this type of hydraulic arrangement involves starting of the machine when temperatures are low. Specifically, as the starter is activated to crank the engine and to start the hydraulic pumps, the starter is required to overcome the drag torque of the engine and the pumps. One component of the drag torque of the pumps is due to the displacement provided by the pump as fluid is moved from the storage tank to the hydraulic actuators. Another component of the drag torque of the pumps is due to the drag on rotating groups or elements or other moving parts within the pump caused by fluid which is not being pumped but which fills the pump casing to provide lubrication. The drag torque of the pumps may be particularly high when the hydraulic fluid is at low temperature due to the enhanced viscosity of the hydraulic fluid being pumped and filling the pump case in surrounding relation to moving parts. Thus, a system to reduce drag torque of the hydraulic pumps in cold conditions may be useful.
Prior practices used to reduce the torque drag of the hydraulic pumps typically have focused on adjustment of the pump displacement to reduce initial torque drag during start-up. One such practice is described in U.S. Pat. No. 3,522,999 issued to Liles on Aug. 4, 1970. Specifically, this patent describes the use of a bypass valve to direct fluid being pumped from the high pressure outlet side of the system to the low pressure inlet side of the system until the engine reaches its idling speed and the temperature of hydraulic fluid exceeds a defined level. While this approach may provide benefits in reducing torque drag, it does so by reducing the initial effective output of the pump.
The disclosed hydraulic pump system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.